Polypropylene vs. Polyethylene: Material Differences and Comparisons

When comparing polypropylene vs. polyethylene you’ll find they have a lot in common. They find use across many industries, and both polymers have very similar chemical structures. However, there are also some key differences between the two that need to be considered. 

Polypropylene (PP) and polyethylene (PE) both belong to the thermoplastic family. Depending on the grade, they can be melted and reformed multiple times. Polypropylene is a polyolefin, which means that it is made up of repeating units of propylene. Polyethylene, on the other hand, while also a polyolefin, can be made from a variety of different monomers, the most common being ethylene. 

Of the two, polypropylene can withstand higher temperatures. It is more rigid and has a higher melting point than polyethylene. Also, polypropylene is used in food packaging, as it is food-safe and moisture-resistant. Polypropylene finds its way into packaging, carpets, ropes, and plastic parts. 

Polyethylene is widely used in packaging as well since it is very strong and durable. This includes food packaging. Other common places to find polyethylene are cable and wire insulation, toys, buckets, and grocery bags. In this article, we will compare polypropylene vs. polyethylene in terms of its properties, applications, and more.

What is Polypropylene?

Polypropylene (PP) is a thermoplastic polymer that belongs to the polyolefin family. It is made up of repeating units of propylene and can be made through various different manufacturing processes, including injection molding and extrusion. For more information, see our guide on All About Polypropylene.

Figure 1 below are examples of Polypropylene (PP) pellets:

• Atactic (aPP). Irregular methyl group (CH3) arrangement
• Isotactic (iPP). Methyl groups (CH3) arranged on one side of the carbon chain
• Syndiotactic (sPP). Alternating methyl group (CH3) arrangement

PP belongs to the polyolefin family of polymers and is one of the top-three most-used polymers today. Polypropylene has applications—both as a plastic and as a fiber—in the automotive industry, industrial applications, consumer goods, and the furniture market.

Different Types of Polypropylene

Homopolymers and copolymers are the two main types of polypropylene available in the market. 

Propylene homopolymer is the most widely utilized general-purpose grade. It contains only propylene monomer in a semi-crystalline solid form. Main applications include packaging, textiles, healthcare, pipes, automotive, and electrical applications.

Polypropylene copolymer is divided into random copolymers and block copolymers produced by polymerizing of propene and ethane:

1. Propylene random copolymer is produced by polymerizing together ethene and propene. It features ethene units, usually up to 6% by mass, incorporated randomly in the polypropylene chains. These polymers are flexible and optically clear, making them suitable for applications requiring transparency and for products requiring an excellent appearance.
2. Propylene block copolymer contains a higher ethene content (between 5 and 15%). It has co-monomer units arranged in a regular pattern (or blocks). The regular pattern makes the thermoplastic tougher and less brittle than the random co-polymer. These polymers are suitable for applications requiring high strength, such as industrial usages.

Another type of polypropylene is impact copolymer. A propylene homopolymer containing a co-mixed propylene random copolymer phase which has an ethylene content of 45-65% is referred to PP impact copolymer. Impact copolymers are mainly used in packaging, houseware, film, and pipe applications, as well as in the automotive and electrical segments.

Polypropylene suppliers include A. Schulman, Borealis, ExxonMobil Chemical, LyondellBasell, SABIC, and RTP Company. For a full list of commercial grades and PP suppliers, click here.

Polypropylene Homopolymer vs. Polypropylene Copolymer

Propylene homopolymer has a high strength-to-weight ratio, and is stiffer and stronger than the copolymer. These properties combined with good chemical resistance and weldability make it a material of choice in many corrosion resistant structures. 

Polypropylene copolymer is a bit softer but has better impact strength. It’s tougher and more durable than propylene homopolymer. It tends to have better stress crack resistance and lower temperature toughness than homopolymer at the expense of a small reduction in other properties.

PP Homopolymer and PP Copolymer Applications

The applications are nearly identical because of their extensively shared properties. As a result, the choice between these two materials is often made based on non-technical criteria.

Keeping information about the properties of a thermoplastic beforehand is always beneficial. This helps in selecting the right thermoplastic for an application. It also assists in evaluating  the end use requirement would be fulfilled or not. Here are some key properties and benefits of polypropylene:

Melting point of polypropylene. The melting point of polypropylene occurs at a range.

• Homopolymer: 160-165°C
• Copolymer: 135-159°C

Density of polypropylene. PP is one of the lightest polymers among all commodity plastics. This feature makes it a suitable option for lightweight/weight--saving applications.

• Homopolymer: 0.904-0.908 g/cm3
• Random copolymer: 0.904-0.908 g/cm3
• Impact copolymer: 0.898-0.900 g/cm3

Polypropylene chemical resistance

• Excellent resistance to diluted and concentrated acids, alcohols, and bases
• Good resistance to aldehydes, esters, aliphatic hydrocarbons, and ketones
• Limited resistance to aromatic and halogenated hydrocarbons and oxidizing agents

Other values

• Flammability:Polypropylene is a highly flammable material

• PP retains mechanical and electrical properties at elevated temperatures, in humid conditions, and when submersed in water. It is a water-repellent plastic
• PP has good resistance to environmental stress and cracking
• It is sensitive to microbial attacks (bacteria, mold, etc.)
• It exhibits good resistance to steam sterilization

Polymer additives like clarifiers, flame retardants, glass fibers, minerals, conductive fillers, lubricants, pigments, and many other additives can further improve PP’s physical and/or mechanical properties. For example, PP has poor resistance to UV, hence light stabilization with hindered amines enhances the service life as compared to unmodified polypropylene.